Ink jet recording method and ink jet recording apparatus

ABSTRACT

Provided is an ink jet recording method including: discharging an ultraviolet ray-curable ink, which has a viscosity at 20° C. of 25 mPa·s or less and an average equivalent of polymerizable unsaturated double bond of 100 to 150, from a head onto a recording medium at a discharge temperature of 30° C. to 40° C.; and irradiating the ultraviolet ray-curable ink, attached onto the recording medium, with ultraviolet rays emitted from a light source to cure the ultraviolet ray-curable ink.

BACKGROUND 1. Technical Field

The present invention relates to an ink jet recording method and an inkjet recording apparatus.

2. Related Art

In the related art, various methods are used as a recording method offorming an image on a recording medium such as paper based on image datasignals. Among these methods, when an ink jet method is used, ink can bedischarged onto only a desired image portion of a recording medium withan inexpensive apparatus and an image is directly formed on therecording medium. Therefore, ink can be efficiently used and runningcost is low. Furthermore, little noise is made in the ink jet method,which is superior as a recording method.

Recently, an ink jet recording method using a photocurable ink in whichmonomers are photopolymerized (cured) by irradiation of light has beendisclosed. With this ink jet method, an image having superior waterresistance and wear resistance can be formed on a recording surface of arecording medium. Therefore, this ink jet method is used for manufactureof a color filter; printing (recording) on a printed circuit board, aplastic card, a plastic sheet, a large-sized signboard, and a plasticcomponent; and printing of a barcode or a date.

For example, Japanese Patent No. 4335955 discloses an energy ray-curableink composition including a coloring material, a polymerizable compound,a photopolymerization initiator, and a surface conditioner. In the inkcomposition, the polymerizable compound consists only of amonofunctional monomer having an acrylic equivalent of 300 or less, andhaving one ethylenic double bond in one molecule, and a polyfunctionalmonomer having an acrylic equivalent of 150 or less, and having two ormore ethylenic double bonds in one molecule; the photopolymerizationinitiator contains an α-aminoalkylphenone compound and a thioxanthonecompound; and the surface conditioner contains a silicone compoundhaving a polydimethylsiloxane structure. In addition, Japanese PatentNo. 4335955 also discloses the sentence “since the ink compositioncontains the polymerizable compound consisting only of a monofunctionalmonomer having an acrylic equivalent of 300 or less and a polyfunctionalmonomer having an acrylic equivalent of 150 or less, a highly reactiveink composition which has a low viscosity suitable for an ink jetrecording system can be obtained” (paragraphs [0006] and [0007] ofJapanese Patent No. 4335955).

However, when ink jet recording is performed using the energyray-curable ink composition disclosed in Japanese Patent No. 4335955,there are problems in that curability and discharge stabilitydeteriorate; and that image quality stability deteriorates due to asignificant increase in the internal temperature of a recordingapparatus after continuous printing (continuous discharge).

SUMMARY

An advantage of some aspects of the invention is to provide an ink jetrecording method which has effects of obtaining superior curability anddischarge stability; and suppressing an increase in the internaltemperature of a recording apparatus after continuous printing.

The present inventors have thoroughly studied, thereby obtaining thefollowing findings. When a photocurable ink containing a polymerizablecompound is irradiated with light, the polymerizable compound isphotopolymerized and the photocurable ink is fixed (cured). At thistime, the photopolymerization generates reaction heat. However, in theenergy ray-curable ink composition disclosed in Japanese Patent No.4335955, in order to obtain a highly reactive ink having a lowviscosity, the acrylic equivalent of the polymerizable compound, whichis one component, is specified. However, since the acrylic equivalent ofthe entire ink composition is not specified, reaction heat generatedduring the curing of ink is not considered. Accordingly, when ink jetrecording is performed using the ink composition disclosed in JapanesePatent No. 4335955, the amount of reaction heat is increased over time,the internal temperature of an ink jet recording apparatus issignificantly increased, and thus the viscosity of the ink compositionis changed when being discharged. As a result, ink application positionsor the amount of ink discharged may be changed and thus the imagequality of an obtained image is not stable in terms of graininess, hue,and the like. As described above, when the ink composition disclosed inJapanese Patent No. 4335955 is used for a relatively long-periodrecording such as continuous printing, it is difficult to satisfysuperior curability and discharge stability at the same time and toperform recording.

The present inventors have repeatedly discussed about theabove-described reaction heat, thereby obtaining the following findings.First, in order to suppress the amount of reaction heat, it is necessarythat the viscosity of an ink be increased. To that end, it is necessarythat an ink discharged from a head be heated to increase the temperatureof the ink discharged from the head (hereinafter, also referred to as a“discharge temperature”). However, in this case, there is a problem inthat the internal temperature of an ink jet recording apparatus issignificantly increased. On the other hand, when the viscosity of an inkis reduced in order to suppress the heating temperature of the inkdischarged from the head to a relatively low degree, the amount ofreaction heat generated during curing is increased. This case also has aproblem in that the internal temperature of the ink jet recordingapparatus is significantly increased.

Based on the above-described findings, the present inventors havethoroughly discussed about the problems caused by the reaction heat andfound that high-quality images could be stably obtained by stablymaintaining the internal temperature, in particular, the dischargetemperature of an ink jet recording apparatus at a relatively lowtemperature. Specifically, it was found that the amount of reaction heatgenerated during curing could be sufficiently suppressed by maintainingthe discharge temperature in a range of 30° C. to 40° C. Furthermore, inorder to obtain superior curability and discharge stability at the sametime in an ink jet recording method, the present inventors haverepeatedly discussed about a reduction in the viscosity of an ink. As aresult, the present inventors found that the above-described problemscould be solved using an ink jet recording method including: dischargingan ultraviolet ray-curable ink, which has a viscosity at 20° C. of 25mPa·s or less and an average equivalent of polymerizable unsaturateddouble bond of 100 to 150, from a head onto a recording medium at adischarge temperature of 30° C. to 40° C.; and irradiating theultraviolet ray-curable ink, attached onto the recording medium, withultraviolet rays emitted from a light source to cure the ultravioletray-curable ink, thereby completing the invention.

That is, the invention has adopted the following means.

[1] An ink jet recording method including: discharging an ultravioletray-curable ink, which has a viscosity at 20° C. of 25 mPa·s or less andan average equivalent of polymerizable unsaturated double bond of 100 to150, from a head onto a recording medium at a discharge temperature of30° C. to 40° C.; and irradiating the ultraviolet ray-curable ink,attached onto the recording medium, with ultraviolet rays emitted from alight source to cure the ultraviolet ray-curable ink.

[2] The ink jet recording method according to [1], wherein the head is aline head having a length longer than or equal to a width of therecording medium, and recording is performed using a line type ink jetrecording apparatus that discharges the ultraviolet ray-curable ink fromthe line head onto the recording medium which is scanned relative to theline head.

[3] The ink jet recording method according to [1] or [2], wherein asupport supports the recording medium, moves along with the transport ofthe recording medium, and passes through a position opposite the head,and a time period in which the support moves from a position and returnsto the position is longer than or equal to 5 seconds.

[4] The ink jet recording method according to [3], wherein a material ofthe support for the recording medium is a metal.

[5] The ink jet recording method according to any one of [1] to [4],wherein a viscosity at 20° C. of the ultraviolet ray-curable ink is 15mPa·s to 25 mPa·s.

[6] The ink jet recording method according to any one of [1] to [5],wherein the light source is a light emitting diode.

[7] The ink jet recording method according to any one of [1] to [6],wherein the ultraviolet ray-curable ink contains 30% by mass to 70% bymass of monofunctional (meth)acrylate and 20% by mass to 60% by mass ofbifunctional or higher polyfunctional (meth)acrylate.

[8] The ink jet recording method according to any one of [1] to [7],wherein the ultraviolet ray-curable ink is curable by irradiation ofultraviolet rays having an irradiation energy of 200 mJ/cm² or lower.

[9] An ink jet recording apparatus which performs recording using theink jet recording method according to any one of [1] to [8].

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a block diagram illustrating a configuration example of an inkjet recording apparatus according to an embodiment of the invention.

FIG. 2 is a cross-sectional view schematically illustrating an exampleof a configuration in the vicinity of a head unit, a transport unit, andan irradiation unit of a line printer which is an example of an ink jetrecording apparatus according to an embodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of the invention will be described in detail.

In the specification, “recorded material” refers to a recording mediumon which a cured material is formed using an ink. The cured materialdescribed in this specification refers to a cured material containing acured film or a coating film.

In addition, in this specification, “curing” refers to irradiating anink containing a polymerizable compound with light to polymerize thepolymerizable compound and to fix the ink. “Curability” refers to theproperty of being cured in response to light; and is also called“photopolymerizability”. “Curing wrinkles” refers to wrinkles that aregenerated on a surface of a coating film after curing, as a result of ahigh volumetric polymerization shrinkage increased, for example, when anuncured ink in a coating film, which is a target for curing, irregularlyflows before curing.

In addition, in the specification, “discharge stability” refers to theproperty of continuously stably discharging ink droplets from nozzleswithout nozzle clogging.

In addition, in this specification, “storage stability” refers to theproperty of a viscosity of an ink being not easily changed before andafter the ink is stored. “wear resistance” refers to the property of acured material being not easily peeled off or worn off when beingrubbed.

In addition, in this specification, “(meth)acrylate” represents at leastone of acrylate and corresponding methacrylate; “(meth)acryl” representsat least one of acryl and corresponding methacryl; and “(meth)acryloyl”represents at least one of acryloyl and corresponding methacryloyl.

Ink Jet Recording Method

An embodiment of the invention relates to an ink jet recording method(hereinafter, also simply referred to as a “recording method”). Therecording method includes at least a discharging step of discharging anultraviolet ray-curable ink, which has a viscosity at 20° C. in apredetermined range and an average equivalent of polymerizableunsaturated double bond in a predetermined range, from a head onto arecording medium at a discharge temperature in a predetermined range;and a curing step of irradiating the ultraviolet ray-curable ink,attached onto the recording medium, with ultraviolet rays emitted from alight source to cure the ultraviolet ray-curable ink. In this way, theultraviolet ray-curable ink is cured and a cured material thereof isformed on a recording medium.

Hereinafter, first, the ultraviolet ray-curable ink (hereinafter alsoreferred to as an “ink” or an “ink composition”) will be described indetail, and then the respective steps included in the recording methodwill be described.

The ultraviolet ray-curable ink according to the embodiment can be usedfor the above-described ink jet recording method and an ink jetrecording apparatus described below. As described above, the ultravioletray-curable ink has a viscosity at 20° C. and an average equivalent ofpolymerizable unsaturated double bond in the predetermined ranges,respectively.

1. Viscosity at 20° C. of Ultraviolet Ray-Curable Ink

The viscosity at 20° C. of the ultraviolet ray-curable ink is less thanor equal to 25 mPa·s, preferably 15 mPa·s to 25 mPa·s, and morepreferably 17 mPa·s to 23 mPa·s. When the viscosity at 20° C. is lessthan or equal to the above-described upper limit, the ink dischargestability is superior. When the viscosity at 20° C. is greater than orequal to the above-described lower limit, curing wrinkles can beeffectively suppressed.

As described below in Examples, the viscosity of the ink according tothe embodiment can be measured using an E-type viscometer. When theE-type viscometer is used, the fact that the measurement should beperformed according to the instruction manual belongs to the commonknowledge. Accordingly, needless to say, the kinds and rotating speed ofa rotor should be set according to the instruction manual such that theviscosity of the ink, which is a measurement target, can be measurednormally. Therefore, it is also apparent that, in this embodiment,measurement conditions are set according to the instruction manual suchthat the viscosity of the ink, which is a measurement target, can benormally measured normally.

The reason why curing wrinkles are generated is assumed to be asfollows, but the scope of the embodiment is not limited to thisassumption. It is assumed that curing wrinkles are generated in thefollowing cases: in a coating film of the ink, when a surface of thecoating film is cured first and then the inside of the coating film iscured, the surface of the coating film which is cured first may bedeformed; and the ink in the coating film may irregularly flow beforecuring the inside of the coating film. In addition, an ultravioletray-curable ink having a low viscosity has a tendency to have a largepolymerization shrinkage when being cured (the difference between avolume of an ink having a predetermined mass before curing and a volumeof the ink (cured material) after curing). For this reason, it isassumed that curing wrinkles are significantly generated. In addition,an ultraviolet ray-curable ink, which contains a monofunctional(meth)acrylate described below, in particular, a vinyl ethergroup-containing (meth)acrylate represented by a formula (I) describedbelow, has a tendency that curing wrinkles are likely to be generated.In particular, in an ultraviolet ray-curable ink, which contains thevinyl ether group-containing (meth)acrylic acid ester represented by theformula (I) and has a low viscosity, it is assumed that curing wrinklesare significantly generated. In the case of an ultraviolet ray-curableink used in the ink jet recording method according to the embodiment,even when the ink contains the above-described (meth)acrylate, curingwrinkles can be effectively suppressed by setting the viscosity of theink in the above-described range. In this specification, as theviscosity, values measured according to the method used in Examplesbelow can be used.

An example of an ink design method for setting the viscosity of the inkin a desired range will be described.

The viscosity of a mixture of polymerizable compounds included in theink can be calculated from the viscosity of each polymerizable compoundused and the mass ratio of the polymerizable compound to the inkcomposition.

It is assumed that an ink contains N kinds of polymerizable compounds A,B, . . . , N. The viscosity of the polymerizable compound A isrepresented by VA, and the mass ratio of the polymerizable compound A tothe total amount of all the polymerizable compounds in the ink isrepresented by MA. The viscosity of the polymerizable compound B isrepresented by VB, and the mass ratio of the polymerizable compound B tothe total amount of all the polymerizable compounds in the ink isrepresented by MB. Likewise, the viscosity of the polymerizable compoundN is represented by VN, and the mass ratio of the polymerizable compoundN to the total amount of all the polymerizable compounds in the ink isrepresented by MN. Specifically, the expression “MA+MB+ . . . +MN=1” issatisfied. In addition, the viscosity of a mixture of all thepolymerizable compounds included in the ink is represented by VX. Inthis case, it is assumed that the following expression (1) is satisfied.

MA×Log VA+MB×Log VB+ . . . +MN×Log VN=Log VX  (1)

For example, when the ink contains two kinds of polymerizable compounds,the mass ratios of the polymerizable compounds after MB are set to zero.The number of kinds of polymerizable compounds can be set to anarbitrary number of 1 or more.

An example of a procedure (Steps 1 to 7) for setting the viscosity ofthe ink in a desired range will be described.

First, information regarding the viscosity at a predeterminedtemperature of each polymerizable compound to be used is acquired (Step1). For example, the information can be acquired from a manufacturercatalog or the like; or can be acquired by measuring the viscosity ofeach polymerizable compound at a predetermined temperature. Theviscosity of a single polymerizable compound may vary depending onmanufacturers even when the same kind of polymerizable compound is used.Therefore, it is preferable that the viscosity information be acquiredfrom a manufacturer of a polymerizable compound to be used.

Next, VX is set to a target viscosity, and the composition ratio (massratio) of each polymerizable compound is determined based on theexpression (1) such that VX is the target viscosity (Step 2). The targetviscosity is the viscosity of a finally obtained ink composition and isin a range of, for example, 15 mPa·s to 25 mPa·s. The predeterminedtemperature is set to 20° C.

Next, polymerizable compounds are actually mixed with each other toprepare a composition of the polymerizable compounds (hereinafter, alsoreferred to as a “polymerizable composition), and then the viscositythereof at a predetermined temperature is measured (Step 3).

When the viscosity of the polymerizable composition approximatelyapproaches the above-described target viscosity (In Step 4, “targetviscosity±5 mPa·s” is allowable), an ink composition that contains thepolymerizable composition and components other than polymerizablecompounds such as a photopolymerization initiator and a pigment(hereinafter, also referred to as “components other than polymerizablecompounds”) is prepared, and the viscosity of the ink composition ismeasured (Step 4). In Step 4, among the components other than thepolymerizable, for example, when a composition is added to the inkcomposition in the form of a pigment dispersion as in the case of apigment, polymerizable compounds which are contained in the pigmentdispersion in advance are also added to the ink composition. Therefore,it is necessary that the mass ratio of each polymerizable compound inthe ink composition be adjusted to a value obtained by subtracting themass ratio of each polymerizable compound, added to the ink compositionin the form of the pigment dispersion, from the composition ratio ofeach polymerizable compound determined in step 2.

Next, a difference between the measured viscosity of the ink compositionand the measured viscosity of the polymerizable composition iscalculated and is set to VY (Step 5). Usually, “VY>0” is satisfied. VYdepends on containing conditions such as the kind and content of thecomponents other than polymerizable compounds. In Examples below, VY=3mPa·s to 5 mPa·s.

Next, VX is set to a value of “target viscosity of Step 2—VY”, and thecomposition ratio of each polymerization compound is determined againbased on the expression (1) such that VX is the value of “targetviscosity of Step 2—VY” (Step 6).

Next, polymerization compounds having the composition ratios determinedin Step 6 are mixed with components other than polymerizable compoundsto prepare an ink composition, and the viscosity thereof at apredetermined temperature is measured (Step 7). When the measuredviscosity is the target viscosity, the ink composition prepared in Step7 is the ink composition having the target viscosity.

On the other hand, in Step 3 when the measured viscosity of the preparedcomposition of polymerizable compounds is out of the range of “targetviscosity±5 mPa·s”, the following fine adjustment is performed. Then,the process is repeated from Step 3. First, when the measured viscosityis too high, the content of a polymerizable compound having a higherviscosity than the target viscosity is reduced; and the content of apolymerizable compound having a lower viscosity than the targetviscosity is increased. On the other hand, when the measured viscosityis too low, the content of a polymerizable compound having a lowerviscosity than the target viscosity is reduced; and the content of apolymerizable compound having a higher viscosity than the targetviscosity is increased. In addition, in Step 7, when the measuredviscosity of the prepared ink composition is not the target viscosity,the same adjustment as the above-described fine adjustment is performed.Then, the process is repeated from Step 7.

2. Average Equivalent of Polymerizable Unsaturated Double Bond ofUltraviolet Ray-Curable Ink

The ultraviolet ray-curable ink has an average equivalent ofpolymerizable unsaturated double bond of 100 to 150, preferably 110 to150, and more preferably 120 to 150. When the average equivalent ofpolymerizable unsaturated double bond is greater than or equal to theabove-described lower limit, the amount of reaction heat generatedduring curing is suppressed. Therefore, an increase in temperature aftercontinuous printing can be suppressed and storage stability is superior.In addition, when the average equivalent of polymerizable unsaturateddouble bond is less than or equal to the above-described upper limit,curability is superior.

A compound having a polymerizable unsaturated double bond can bereferred to as a compound having a polymerizable functional group whichcontains a polymerizable unsaturated double bond. Examples thereofinclude (meth)acrylate compounds, vinyl compounds, vinyl ethercompounds, and allyl compounds. However, the compound is not limited tothese examples. The compound having a polymerizable unsaturated doublebond is not particularly limited as long as it has one or morepolymerizable functional groups. When the number of polymerizablefunctional groups is more than or equal to 2, the kinds of thepolymerizable functional groups may be the same as or different fromeach other. In addition, based on a structure thereof other than theabove-described polymerizable functional group, the above-describedcompound can be divided into polymerizable compounds having an aromaticring structure, polymerizable compounds having a cyclic or linearaliphatic structure, and polymerizable compounds having a heterocyclicstructure.

In this specification, the average equivalent of polymerizableunsaturated double bond of the ultraviolet ray-curable ink can bemeasured as follows. First, the equivalent of polymerizable unsaturateddouble bond of each polymerizable compound included in the ink iscalculated according to the following expression (2).

Equivalent of Polymerizable Unsaturated Double Bond of PolymerizableCompound=Molecular Weight of Polymerizable Compound/Number ofPolymerizable Unsaturated Double Bonds Included in Molecules ofPolymerizable Compound  (2)

In the expression (2), the molecular weight of a polymerizable compoundand the number of polymerizable unsaturated double bonds can be acquiredfrom values of a manufacturer catalog or values calculated from achemical structural formula thereof.

Next, the average equivalent of polymerizable unsaturated double bond ofthe ink is calculated according to the following expression (3).

Average Equivalent of Polymerizable Unsaturated Double Bond ofInk=(Equivalent of Polymerizable Unsaturated Double Bond ofPolymerizable Compound A×Content of Polymerizable Compound A inInk+Equivalent of Polymerizable Unsaturated Double Bond of PolymerizableCompound B×Content of Polymerizable Compound B in Ink . . . Equivalentof Polymerizable Unsaturated Double Bond of Polymerizable Compoundn×Content of Polymerizable Compound n in Ink)/(Content of PolymerizableCompound A in Ink+Content of Polymerizable Compound B in Ink+ . . .+Content of Polymerizable Compound n in Ink)  (3)

The expression (3) is under the assumption that the ink contains n kindsof polymerizable compounds in which “n” represents an integer of 1 ormore. In the expression (3), “content” is represented by % by mass withrespect to the total mass of the ink.

As the average equivalent of polymerizable unsaturated double bond ofthe ink is lower, the ink contains more polymerizable unsaturated doublebonds, and the amount of reaction heat generated during thepolymerization of the ink is increased. On the other hand, as theaverage equivalent of polymerizable unsaturated double bond of the inkis higher, the ink contains less polymerizable unsaturated double bonds,and the amount of reaction heat generated during the polymerization ofthe ink is reduced.

Hereinafter, additives (components) which can be added to theultraviolet ray-curable ink according to the embodiment will bedescribed.

3. Polymerizable Compound

A polymerizable compound included in the ink can be polymerized alone oralong with the reaction of a photopolymerization initiator byirradiation of light to cure the printed ink. As the polymerizablecompound, various kinds of monofunctional, bifunctional, andtrifunctional or higher polyfunctional monomers and oligomers can beused. Examples of the monomers include unsaturated carboxylic acids suchas (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid,and maleic acid and salts or esters thereof; urethanes; amides andanhydrides thereof; acrylonitriles; styrenes; and various kinds ofunsaturated polyesters, unsaturated polyethers, unsaturated polyamides,and unsaturated urethanes. In addition, examples of the oligomersinclude oligomers derived from the monomers such as a linear acryloligomer; epoxy (meth)acrylate; oxetane (meth)acrylate; cyclic or linearaliphatic urethane (meth)acrylate; aromatic urethane (meth)acrylate; andpolyester (meth)acrylate.

Among these, (meth)acrylic acid esters, that is, (meth)acrylates arepreferable. Among the (meth)acrylates, it is preferable that amonofunctional (meth)acrylate and a bifunctional or higherpolyfunctional (meth)acrylate be used in combination; and it is morepreferable that a monofunctional (meth)acrylate and a bifunctionalpolyfunctional (meth)acrylate be used in combination.

Hereinafter, the polymerizable compound will be described in detailfocusing on (meth)acrylate. Among the above-described monofunctional(meth)acrylates, for example, a vinyl ether group-containing(meth)acrylic acid ester represented by the formula (I) are preferablyused. Therefore, first, the vinyl ether group-containing (meth)acrylicacid ester will be described.

3-1. Vinyl Ether Group-Containing (Meth)Acrylic Acid Ester

It is preferable that the ink contain a vinyl ether group-containing(meth)acrylic acid ester represented by the formula (I).

CH₂═CR¹—COOR²—O—CH═CH—R³  (I)

(wherein R¹ represents a hydrogen atom or a methyl group; R² representsa divalent organic residue having 2 to 20 carbon atoms; and R³represents a hydrogen atom or a monovalent organic residue having 1 to11 carbon atoms)

When the ink contains the vinyl ether group-containing (meth)acrylicacid ester, the viscosity of the ink can be reduced, the curability ofthe ink can be improved, and curing wrinkles can be effectivelysuppressed. Furthermore, a case of using a compound having both a vinylether group and a (meth)acrylic group in one molecule is more preferableto a case of separately using a compound having a vinyl ether group anda compound having a (meth)acrylic group, from the viewpoint of improvingthe curability of the ink.

In the formula (I), preferable examples of the divalent organic residuehaving 2 to 20 carbon atoms represented by R² include an linear,branched, or cyclic alkylene group having 2 to 20 carbon atoms which maybe substituted; an alkylene group having 2 to 20 carbon atoms and havingan oxygen atom of an ether bond or an ester bond in a structure thereofwhich may be substituted; and a divalent aromatic group having 6 to 11carbon atoms which may be substituted. Among these, an alkylene grouphaving 2 to 6 carbon atoms such as an ethylene group, an n-propylenegroup, an isopropylene group, or a butylene group; and an alkylene grouphaving 2 to 9 carbon atoms and an oxygen atom of an ether bond in astructure thereof such as an oxyethylene group, an oxy-n-propylenegroup, an oxyisopropylene group, or an oxybutylene group are preferablyused.

In the formula (I), preferable examples of the monovalent organicresidue having 1 to 11 carbon atoms represented by R³ include a linear,branched, or cyclic alkyl group having 1 to 10 carbon atoms which may besubstituted; and an aromatic group having 6 to 11 carbon atoms which maybe substituted. Among these, an alky group having 1 or 2 carbon atomssuch as a methyl group or an ethyl group; and an aromatic group having 6to 8 carbon atoms such as a phenyl group or a benzyl group arepreferably used.

When each organic residue described above is a groups which may besubstituted, examples of a substituent thereof are divided into a grouphaving carbon atoms and a group not having carbon atoms. First, when thesubstituent is a group having carbon atoms, the number of the carbonatoms corresponds to the number of carbon atoms contained in the organicresidue. Examples of the group having carbon atoms include a carboxylicgroup and an alkoxy group, but the group having carbon atoms is notlimited thereto. Next, examples of the group not having carbon atomsinclude a hydroxyl group and a halo group, but the group not havingcarbon atoms is not limited thereto.

Examples of the vinyl ether group-containing (meth)acrylic acid esterinclude 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate,1-methyl-2-vinyloxyethyl (meth) acrylate, 2-vinyloxypropyl(meth)acrylate, 4-vinyloxybutyl (meth)acrylate,1-methyl-3-vinyloxypropyl (meth) acrylate, 1-vinyloxymethylpropyl (meth)acrylate, 2-methyl-3-vinyloxypropyl (meth) acrylate,1,1-dimethyl-2-vinyloxyethyl (meth) acrylate, 3-vinyloxybutyl(meth)acrylate, 1-methyl-2-vinyloxypropyl (meth)acrylate,2-vinyloxybutyl (meth) acrylate, 4-vinyloxycyclohexyl (meth)acrylate,6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth) acrylate,2-vinyloxymethylcyclohexylmethyl (meth) acrylate,p-vinyloxymethylphenylmethyl (meth) acrylate,m-vinyloxymethylphenylmethyl (meth) acrylate,o-vinyloxymethylphenylmethyl (meth) acrylate, 2-(vinyloxyethoxy)ethyl(meth) acrylate, 2-(vinyloxyisopropoxy)ethyl (meth) acrylate,2-(vinyloxyethoxy)propyl (meth) acrylate, 2-(vinyloxyethoxy) isopropyl(meth) acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyethoxy)ethyl (meth) acrylate,2-(vinyloxyethoxyisopropoxy)ethyl (meth)acrylate,2-(vinyloxyisopropoxyethoxy)ethyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)ethyl (meth)acrylate,2-(vinyloxyethoxyethoxy)propyl (meth) acrylate,2-(vinyloxyethoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxyethoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate,2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate,2-(isopropenoxyethoxy)ethyl (meth) acrylate,2-(isopropenoxyethoxyethoxy)ethyl (meth) acrylate,2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate,2-(isopropenoxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate,polyethylene glycol monovinyl ether (meth)acrylate, and polypropyleneglycol monovinyl ether (meth)acrylate. However, the vinyl ethergroup-containing (meth)acrylic acid ester is not limited thereto.

Among these, from the viewpoints of reducing the viscosity of the ink,increasing the flash point, and improving the curability of the ink,2-(vinyloxyethoxy)ethyl (meth)acrylate, that is, at least one of2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethylmethacrylate is preferable; and 2-(vinyloxyethoxy)ethyl acrylate is morepreferable. In particular, both 2-(vinyloxyethoxy)ethyl acrylate and2-(vinyloxyethoxy)ethyl methacrylate have a simple structure and a lowmolecular weight, the viscosity of the ink can be significantly reduced.Examples of 2-(vinyloxyethoxy)ethyl (meth)acrylate include2-(2-vinyloxyethoxy)ethyl (meth)acrylate and 2-(1-vinyloxyethoxy)(meth)acrylate. Examples of 2-(vinyloxyethoxy)ethyl acrylate include2-(2-vinyloxyethoxy)ethyl acrylate and 2-(1-vinyloxyethoxy) acrylate.2-(vinyloxyethoxy)ethyl acrylate is superior to 2-(vinyloxyethoxy)ethylmethacrylate from the viewpoints of curability.

As the vinyl ether group-containing (meth)acrylic acid ester, one kindmay be used alone or two or more kinds may be used in combination.

Examples of a method of preparing the vinyl ether group-containing(meth)acrylic acid ester include a method (preparation method B) ofesterifying (meth)acrylic acid with hydroxyl group-containing vinylether, a method (preparation method C) of esterifying (meth)acrylic acidhalide with hydroxyl group-containing vinyl ether, a method (preparationmethod D) of esterifying (meth)acrylic anhydride with hydroxylgroup-containing vinyl ether, a method (preparation method E) ofester-exchanging (meth)acrylate with hydroxyl group-containing vinylether, a method (preparation method F) of esterifying (meth)acrylic acidwith halogen-containing vinyl ether, a method (preparation method G) ofesterifying (meth)acrylic acid alkali (earth) metal salt withhalogen-containing vinyl ether, a method (preparation method H) ofvinyl-exchanging hydroxyl group-containing (meth)acrylate with vinylcarboxylate, and a method (preparation method I) of ether-exchanginghydroxyl group-containing (meth)acrylate with alkyl vinyl ether.However, the method of preparing vinyl ether group-containing(meth)acrylic acid ester is not limited thereto.

Among these methods, the preparation method E is preferable from theviewpoint of obtaining superior desired effects of the embodiment.

3-2. Monofunctional (Meth)acrylate

It is preferable that the ink contain a monofunctional (meth)acrylate.When the ink contains the above-described vinyl ether group-containing(meth)acrylic acid ester (which is limited to a monofunctional(meth)acrylate), the above-described monofunctional (meth)acrylateincludes the vinyl ether group-containing (meth)acrylic acid ester. Thedescription of the vinyl ether group-containing (meth)acrylic acid esterwill not be repeated. Hereinafter, monofunctional (meth)acrylates otherthan the above-described vinyl ether group-containing (meth)acrylic acidester will be described. When the ink contains the monofunctional(meth)acrylate, the viscosity of the ink can be reduced, the curabilityof the ink is further improved, and the solubility of aphotopolymerization initiator and other additives is superior.Furthermore, since the solubility of a photopolymerization initiator andother additives is superior, the ink discharge stability is furtherimproved and the toughness, heat resistance, and chemical resistance ofa coating film are improved.

Examples of the monofunctional (meth)acrylate include phenoxyethyl(meth)acrylate, isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl(meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomyristyl(meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol(meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl(meth)acrylate, 2-methoxyethyl (meth)acrylate, butoxyethyl(meth)acrylate, ethoxy diethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxy polyethylene glycol(meth)acrylate, methoxy propylene glycol (meth) acrylate,tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,2-hydroxy-3-phenoxypropyl (meth) acrylate, lactone-modified flexible(meth) acrylate, t-butylcyclohexyl (meth) acrylate, dicyclopentanyl(meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, benzyl(meth)acrylate, ethoxylated nonylphenyl (meth) acrylate, alkoxylatednonylphenyl (meth)acrylate, and p-cumylphenol EO-modified(meth)acrylate.

Among these, from the viewpoints of further improving curability,storage stability, and the solubility of a photopolymerizationinitiator, a monofunctional (meth)acrylate having an aromatic ringstructure in a molecule is preferable. Preferable examples of themonofunctional (meth)acrylate having an aromatic ring structure includephenoxyethyl (meth)acrylate, benzyl (meth) acrylate,2-hydroxyphenoxypropyl (meth)acrylate, and phenoxy diethylene glycol(meth)acrylate. However, the monofunctional (meth)acrylate having anaromatic ring structure is not limited thereto. Among these, at leastone of phenoxyethyl (meth)acrylate and benzyl (meth)acrylate ispreferable; and phenoxyethyl (meth)acrylate is more preferable, from theviewpoints of reducing the viscosity of the ink and obtaining superiorcurability, wear resistance, the adhesion of the ink on a recordingmedium, and the solubility of a photopolymerization initiator at thesame time.

As the monofunctional (meth)acrylate other than the vinyl ethergroup-containing (meth)acrylic acid ester, one kind may be used alone ortwo or more kinds may be used in combination.

The content of the monofunctional (meth)acrylate is preferably 30% bymass to 70% by mass and more preferably 40% by mass to 60% by mass withrespect to the total mass (100% by mass) of the ink. When the content isin the above-described range, the viscosity, specifically, both theviscosity at 20° C. the viscosity at a discharge temperature of the inkare easily adjusted in desired ranges. Furthermore, when the content isgreater than or equal to the upper limit, curability is further improvedand the solubility of a photopolymerization initiator is also superior.Meanwhile, when the content is less than or equal to the above-describedupper limit, curability is further improved and adhesion is alsosuperior.

When the ink contains the vinyl ether group-containing (meth)acrylicacid ester which is a monofunctional (meth)acrylate, the content of themonofunctional (meth)acrylate includes the content of the vinyl ethergroup-containing (meth)acrylic acid ester.

In particular, when the ink contains the vinyl ether group-containing(meth)acrylic acid ester, the content of the vinyl ethergroup-containing (meth)acrylic acid ester is preferably 10% by mass to50% by mass and more preferably 15% by mass to 40% by mass with respectto the total mass (100% by mass) of the ink. When the content is greaterthan or equal to the above-described lower limit, the viscosity of theink can be reduced and the curability of the ink can be furtherimproved. Meanwhile, when the content is less than or equal to theabove-described upper limit, the storage stability of the ink can bemaintained in a favorable state and curing wrinkles can be furthereffectively suppressed.

In addition, when the ink contains a monofunctional (meth)acrylate otherthan the vinyl ether group-containing (meth)acrylic acid ester, thecontent of the (meth)acrylate is preferably 10% by mass to 40% by massand more preferably 10% by mass to 30% by mass. When the content isgreater than or equal to the above-described lower limit, the solubilityof a photopolymerization initiator as well as curability is furtherimproved. Meanwhile, when the content is less than or equal to theabove-described upper limit, adhesion as well as curability is furtherimproved. As the monofunctional (meth)acrylate other than the vinylether group-contain (meth)acrylic acid ester, a monofunctional(meth)acrylate having an aromatic structure is preferable from theviewpoints of further improving curability and the solubility of aphotopolymerization initiator.

3-3. Bifunctional or Higher Polyfunctional (Meth)acrylate

It is preferable that the ink contain a bifunctional or higherpolyfunctional (meth)acrylate. As described above, it is more preferablethat the monofunctional (meth)acrylate and the bifunctional or higherpolyfunctional (meth)acrylate be used in combination.

Examples of a bifunctional (meth)acrylate include diethylene glycol(meth)acrylate, triethylene glycol (meth) acrylate, tetraethylene glycol(meth) acrylate, polyethylene glycol (meth)acrylate, dipropylene glycol(meth) acrylate, tripropylene glycol (meth) acrylate, polypropyleneglycol (meth) acrylate, 1,4-butandeiol di(meth)crylate, 1,6-hexanedioldi(meth)acrylate, 1,9-nonanediol di(meth)acrylate, neopentyl glycoldi(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO(ethylene oxide) adduct of bisphenol A di(meth)acrylate, PO (propyleneoxide) adduct of bisphenol A di(meth)acrylate, hydroxypivalic acidneopentyl glycol di(meth)acrylate, and polytetramethylene glycol (meth)acrylate.

Examples of a trifunctional or higher (meth)acrylate includetrimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropanetri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate,ditrimethylolpropane tetra(meth)acrylate, glycerin propoxytri(meth)acrylate, caprolactone-modified trimethylolpropanetri(meth)acrylate, pentaerythritolethoxy tetra(meth)acrylate, andcaprolactam-modified dipentaerythritol hexa(meth)acrylate.

As the bifunctional or higher polyfunctional (meth)acrylate, one kindmay be used alone or two or more kinds may be used in combination.

It is preferable that the content of the bifunctional or higherpolyfunctional (meth)acrylate be determined according to therelationship with the preferable content of the monofunctional(meth)acrylate. The content of bifunctional or higher polyfunctional(meth)acrylate is preferably 20% by mass to 60% by mass and morepreferably 20% by mass to 50% by mass with respect to the total mass(100% by mass) Of the ink. When the content is in the above-describedrange, the curability of the ink and the wear resistance of a curedmaterial are superior. As a result, the viscosity of the ink is easilyset to a desired density. In addition, it is preferable that amonofunctional (meth)acrylate having a relatively low viscosity of thepolymerizable compound alone, in particular, the vinyl ethergroup-containing (meth)acrylic acid ester having a particularly lowviscosity; and another polymerizable compound having a relatively highviscosity be used in combination. As a result, the viscosity of the inkis easily set to a desired range.

The total content of polymerizable compounds is preferably 50% by massto 95% by mass with respect to the total content (100% by mass) of theink, in consideration of the relationship with the content of the othercomponents.

In addition, the addition of a photopolymerization initiator can beomitted by using photopolymerizable compounds as the polymerizablecompounds. However, it is preferable that a photopolymerizationinitiator be used because the start of polymerization can be easilyadjusted.

4. Photopolymerization Initiator

The ink according to the embodiment may contain a photopolymerizationinitiator. The photopolymerization initiator is used for curing the inkon a surface of a recording medium by photopolymerization withirradiation of ultraviolet rays and forming an image thereon. By usingultraviolet (UV) rays among light rays, safety can be improved and thecost of a light source lamp can be suppressed. The photopolymerizationinitiator is not particularly limited as long as it generates activespecies such as radicals or cations with irradiation of ultraviolet raysand causes to start the polymerization of the polymerizable compounds.For example, a radical photopolymerization initiator and a cationicphotopolymerization initiator can be used. Among these, a radicalphotopolymerization initiator is preferably used.

Examples of the radical photopolymerization initiator include aromaticketones, acylphosphine oxide compounds, aromatic onium salt compounds,organic peroxides, thio compounds (such as thioxanthone compounds andthiophenyl group-containing compounds), hexaarylbiimidazole compounds,keto oxime ester compounds, borate compounds, adinium compounds,metallocene compounds, active ester compounds, compounds having acarbon-halogen bond, and alkylamine compounds.

Among these, acylphosphine oxide compounds are particularly preferablefrom the viewpoint of further improving the curability of the ink.

Specific examples of the radical photopolymerization initiator includeacetophenone, acetophenonebenzylketal, 1-hydroxycyclohexylphenylketone,2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde,fluorene, anthraquinone, triphenylamine, carbazole,3-methylacetophenone, 4-chlorobenzophenone, 4,4′-dimethoxybenzophenone,4,4′-diaminobenzophenone, Michler's ketone, benzoin propyl ether,benzoin ethyl ether, benzyldimethylketal,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-hydroxy-2-methyl-1-phenylpropan-1-one, thioxanthone,diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone,2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide,2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide,2,4-diethylthioxanthone, andbis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

Examples of commercially available products of the radicalphotopolymerization initiator include IRGACURE 651(2,2-dimethoxy-1,2-diphenylethan-1-one), IRGACURE 184(1-hydroxy-cyclohexyl-phenyl-ketone), DAROCUR 1173(2-hydroxy-2-methyl-1-phenyl-propan-1-one), IRGACURE 2959(1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one),IRGACURE 127(2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phenyl]-2-methyl-propan-1-one),IRGACURE 907 (2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one),IRGACURE 369(2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1), IRGACURE379(2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone),DAROCUR TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), IRGACURE819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 784(bis(η5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium),IRGACURE OXE 01 (1,2-octanedione, 1-[4-(phenylthio)-,2-(O-benzoyloxime)]), IRGACURE OXE 02 (ethanone,1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime)),IRGACURE 754 (mixture of oxyphenylacetic acid,2-[2-oxo-2-phenylacetoxyethoxy]ethylester, oxyphenylacetic acid, and2-(2-hydroxyethoxy)ethylester) (trade names, all of which aremanufactured by BASF Japan Ltd.), KAYACURE DETX-S(2,4-diethylthioxanthone) (trade name, manufactured by Nippon KayakuCo., Ltd.), SPEEDCURE TPO (2,4,6-trimethylbenzoyl-diphenylphosphinoxide), SPEEDCURE DETX (2,4-diethylthioxanthen-9-one) (trade names, allof which are manufactured by Lambson Ltd.), LUCIRIN TPO, LR8893, LR8970(trade names, all of which are manufactured by BASF Japan Ltd.), andUBECRYL P36 (manufactured by UCB Japan Co., Ltd.).

As the photopolymerization initiator, one kind may be used or two ormore kinds may be used in combination. It is preferable that the contentof the photopolymerization initiator be less than or equal to 20% bymass with respect to the total mass (100% by mass) from the viewpointsof improving the curing rate of ultraviolet rays to obtain superiorcurability and of avoiding the remaining of an undissolvedphotopolymerization initiator and coloring caused by thephotopolymerization initiator.

In particular, when the photopolymerization initiator contains anacylphosphine oxide compound, the content thereof is preferably 5% bymass to 15% by mass and more preferably 7% by mass to 13% by mass withrespect to the total mass (100% by mass) of the ink. When the content isgreater than or equal to the above-described lower limit, curability isfurther improved. More specifically, particularly when curing isperformed using an LED (preferably, one having an emission peakwavelength of 360 nm to 420 nm), a curing rate is sufficiently high.Therefore, curability is further improved. Meanwhile, when the contentis less than or equal to the above-described upper limit, the solubilityof the photopolymerization initiator is further improved.

5. Colorant

The ink according to the embodiment may contain a colorant. As thecolorant, at least one of a pigment and a dye can be used.

5-1. Pigment

When a pigment is used as the colorant, the light resistance of the inkcan be improved. As the pigment, both an inorganic pigment and anorganic pigment can be used.

Examples of the inorganic pigment include carbon blacks (C.I. PigmentBlack 7) such as furnace black, lamp black, acetylene black, iron oxide,and titanium oxide.

Examples of the organic pigment include azo pigments such as insolubleazo pigments, condensed azo pigments, azo lakes, and chelate azopigments; polycyclic pigments such as phthalocyanine pigments, peryleneand perinone pigments, anthraquinone pigments, quinacridone pigments,dioxane pigments, thioindigo pigments, isoindolinone pigments, andquinophthalone pigments; dye chelates (for example, basic dye chelatesand acidic dye chelates); dye lakes (for example, basic dye lakes andacidic dye lakes); nitro pigments; nitroso pigments; aniline blacks; anddaylight fluorescent pigments.

Examples of a pigment used for white ink include C.I. Pigment White 6,18, and 21.

Examples of a pigment used for yellow ink include C.I. Pigment Yellow 1,2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55,65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114,117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 167, 172,and 180.

Examples of a pigment used for magenta ink include C.I. Pigment Red 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23,30, 31, 32, 37, 38, 40, 41, 42, 48 (Ca), 48 (Mn), 57 (Ca), 57:1, 88,112, 114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176,177, 178, 179, 184, 185, 187, 202, 209, 219, 224, and 245; and C.I.Pigment Violet 19, 23, 32, 33, 36, 38, 43, and 50.

Examples of a pigment used for cyan ink include C.I. Pigment Blue 1, 2,3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, and 66;and C.I. Vat Blue 4 and 60.

In addition, examples of a pigment used for pigments other than magenta,cyan, and yellow include C.I. Pigment Green 7 and 10; C.I. Pigment Brown3, 5, 25, and 26; and C.I. Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16,24, 34, 36, 38, 40, 43, and 63.

As the pigment, one kind may be used alone or two or more kinds may beused in combination.

When the pigment is used, the average particle size thereof ispreferably less than or equal to 300 nm and more preferably 50 nm to 200nm. When the average particle size is in the above-described range, thereliability in the discharge stability and dispersion stability of theink can be further improved and a high-quality image can be formed. Inthis specification, the average particle size is measured using dynamiclight scattering.

5-2. Dye

As the colorant, a dye can be used. The dye is not particularly limited,and an acid dye, a direct dye, a reactive dye, and a basic dye can beused. Examples of the dye include C.I. Acid Yellow 17, 23, 42, 44, 79,and 142; C.I. Acid Red 52, 80, 82, 249, 254, and 289; C.I. Acid Blue 9,45, and 249; C.I. Acid Black 1, 2, 24, and 94; C.I. Food Black 1 and 2;C.I. Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144, and173; C.I. Direct Red 1, 4, 9, 80, 81, 225, and 227; C.I. Direct Blue 1,2, 15, 71, 86, 87, 98, 165, 199, and 202; C.I. Direct Black 19, 38, 51,71, 154, 168, 171, and 195; C.I. Reactive Red 14, 32, 55, 79, and 249;and C.I. Reactive Black 3, 4, and 35.

As the dye, one kind may be used, or two or more kinds may be used incombination.

The content of the colorant is preferably 1% by mass to 20% by mass withrespect to the total mass (100% by mass) of the ink, from the viewpointsof obtaining superior opacifying properties and color reproduction.

6. Dispersant

When the ink according to the embodiment contains the pigment, adispersant may be added thereto in order to improve pigmentdispersibility. The dispersant is not particularly limited. For example,a well-known dispersant such as a polymeric dispersant which is usuallyused for preparing a pigment dispersant may be used. Specific examplesthereof include those containing, as a major component, one kind or morekinds of polyoxyalkylene polyamine, vinyl-based polymers and copolymers,acrylic polymers and copolymers, polyesters, polyamides, polyimides,polyurethanes, amine-based polymers, silicon-containing polymers,sulfur-containing polymers, fluorine-containing polymers, and epoxyresins. Examples of commercially available products of the polymericdispersant include AJISPER series (trade name, manufactured by AjinomotoFine-Techno Co., Inc.); SOLSPERSE series (32000 and 36000 (trade name)manufactured by Avecia Co.); DISPERBYK series (trade name, manufacturedby BYK Chemie); and DISPARLON series (trade name, manufactured byKusmoto Chemicals Ltd.).

As the dispersant, one kind may be used alone or two or more kinds maybe used in combination. The content of the dispersant is notparticularly limited, and an appropriate amount thereof may be added.

7. Other Additives

The ink according to the embodiment contains other additives(components) other than the above-described additives. These componentsare not particularly limited, and examples thereof include well-knownmaterials of the related art such as a fluorescent brightening agent(sensitizer), a silicone-based surfactant or the like, a polymerizationinhibitor, a polymerization promoter, a penetration enhancer, and awetting agent (moisturizing agent); and other additives. Examples ofother additives include well-known additives of the related art such asa fixing agent, an antifungal agent, a preservative, an antioxidant, anultraviolet absorber, a chelating agent, a pH adjuster, and a thickener.

Next, a recording medium used for the recording method and therespective steps included in the recording method will be described indetail.

8. Recording Medium

Examples of the recording medium include ink non-absorbent or lowink-absorbent recording mediums. Among the recording mediums, examplesof the ink non-absorbent recording mediums include plastic films whichis not subjected to a surface treatment for ink jet recording (that is,on which an ink absorbing layer is not formed); and recording mediumsobtained by coating a substrate such as paper with plastic or by bondinga plastic film thereto. Examples of the plastic described herein includepolyvinyl chloride (PVC), polyethylene terephthalate (PET),polycarbonate (PC), polystyrene (PS), polyurethane (PU), polyethylene(PE), and polypropylene (PP). Examples of the low ink-absorbentrecording mediums include printing paper such as art paper, coatedpaper, and matte paper.

9. Discharging Step

In the discharging step according to the embodiment, the ultravioletray-curable ink is discharged onto from a head onto a recording mediumat a predetermined discharge temperature. The discharge temperature is30° C. to 40° C.

The range of 30° C. to 40° C. is relatively low for a temperatureobtained by heating. In this way, the temperature (dischargetemperature) of the ink to be discharged is relatively low, there aresubstantially no variations in temperature. As a result, an advantageouseffect of improving the ink discharge stability can be obtained.

The discharge temperature described in this specification adopts valuesmeasured as follows. The temperature of a thermocouple, which isprovided on a nozzle surface of a nozzle plate provided in a head, ismeasured before starting printing, and a measured temperature is set tothe discharge temperature. However, the embodiment is not limited tothis method, and any methods of measuring the discharge temperaturewhich may be adopted in the embodiment may be used. In addition, thedischarge temperature of the ink can be set to a desired temperature byadopting a configuration in which an ink heating device is arranged on apath for supplying the ink from an ink cartridge, accommodating the ink,to the head to heat the ink; and the ink heated by the ink heatingdevice is supplied to the head.

Hereinafter, the above-described discharge temperature will be describedin detail. When the temperature is higher than or equal to 30° C.,discharge stability is superior. In addition, the viscosity of anultraviolet ray-curable ink is extremely low at a temperature less than30° C., but there is a problem in that curing wrinkles are easilygenerated due to this low viscosity. On the other hand, the inkaccording to the embodiment can avoid this problem. In particular, theabove-described problem is significantly magnified when a printer is aline printer and when a light source is a light emitting diode (LED).Therefore, in particular, when a line printer or an LED is used in theembodiment, large effects are obtained.

On the other hand, when the discharge temperature is less than or equalto 40° C., an increase in the internal temperature of a recordingapparatus can be suppressed.

In addition, in order to further enhance the above-described effects andto reliably avoid the above-described problems, the dischargetemperature is preferably 34° C. to 40° C.

The viscosity of the ink at the above-described discharge temperature ispreferably 8 mPa·s to 15 mPa·s and more preferably 8 mPa·s to 13 mPa·s.When the viscosity is in the above-described range, curing wrinkles,which may be generated by the composition of the ink, can be effectivelysuppressed; and unstable discharge caused by a high viscosity isprevented and thus discharge stability is further improved.

In addition, as described above, an ultraviolet ray-curable ink has ahigher viscosity that that of an aqueous ink which is normally used asink jet ink. Therefore, the viscosity largely varies depending on thevariation in temperature when being discharged. Such viscosity variationof the ink largely affects on changes in the size and discharge rate ofink droplets, which may lead to deterioration in image quality.Therefore, it is preferable that the temperature (discharge temperature)of the ink to be discharged be maintained as constantly as possible. Thedischarge temperature of the ink according to the embodiment isrelatively low and can be maintained substantially constantly.Therefore, the ink according to the embodiment has superior imagequality stability.

10. Curing Step

In the curing step included in the recording method according to theembodiment, the ultraviolet ray-curable ink, attached onto the recordingmedium, is irradiated with ultraviolet rays (light) emitted from a lightsource to cure the ultraviolet ray-curable ink. In this step, thephotopolymerization initiator included in the ink is decomposed intoinitiating species such as radicals, acids, and bases by irradiation ofultraviolet rays; and the polymerization of polymerizable compounds ispromoted by the function of the initiating species. Alternatively, inthis step, the polymerization of polymerizable compounds starts due toirradiation of ultraviolet rays. At this time, when the ink containsboth a polymerization initiator and a sensitizing dye, the sensitizingdye in the system absorbs ultraviolet rays and is excited. When thisexcited sensitizing dye comes into contact with the photopolymerizationinitiator, the decomposition of the photopolymerization initiator ispromoted, thereby achieving a higher-sensitivity curing reaction.

As the light source (ultraviolet ray source), a mercury lamp or a gas orsolid-state laser is normally used. As a light source used for curingthe ultraviolet ray-curable ink, a mercury lamp and a metal halide lampare widely known. Meanwhile, currently, a mercury-free lamp is stronglydesired from the viewpoint of environmental protection, and it isenvironmentally helpful that a GaN-based ultraviolet light-emittingdevice is used instead. Furthermore, a light emitting diode (LED) suchas an ultraviolet light emitting diode (UV-LED) and an ultraviolet laserdiode (UV-LD) has characteristics of small size, long lifetime, highefficiency, and low cost; and thus is expected as a light source forultraviolet ray-curable ink.

As described above, both an LED and a metal halide lamp are preferablyused as the light source for the ultraviolet ray-curable ink accordingto the embodiment, but an LED is more preferable.

The emission peak wavelength of the light source (ultraviolet raysource) is preferably 360 nm to 420 nm and 380 nm to 410 nm. When theemission peak wavelength is in the above-described range, an UV-LED iseasily available at a low cost, which is preferable.

In addition, the peak intensity (irradiation peak intensity) ofultraviolet rays emitted from a light source (preferably, an LED) havingan emission peak wavelength in the above-described range is preferablygreater than or equal to 500 mW/cm², more preferably greater than orequal to 800 mW/cm², and still more preferably greater than or equal to1,000 mW/cm². When the irradiation peak intensity is in theabove-described range, curability is further improved and curingwrinkles can be effectively suppressed. In particular, when the inkdischarged onto the recording medium is irradiated with ultravioletrays, the initial irradiation peak intensity of the ultraviolet rays isset in the above-described range. As a result, curing wrinkles can beeffectively suppressed. The reason why curing wrinkles are generated isassumed as described above. However, it is assumed that, when theirradiation peak intensity is in the above-described range, both asurface of a coating film and the inside thereof can be simultaneouslycured; and as a result, curing wrinkles can be effectively suppressed.Furthermore, when the viscosity at 20° C. of the ink according to theembodiment is greater than or equal to 15 mPa·s, curing wrinkles can bemore effectively suppressed. In particular, when the ultravioletray-curable ink contains the vinyl ether group-containing (meth)acrylicacid ester represented by the formula (I); and the irradiation peakintensity is in the above-described range, curability is furtherimproved and curing wrinkles are more efficiently suppressed.

The irradiation peak intensity described in this specification adoptsvalues measured using light receiving units UM-10 and UM-400 (both ofwhich are manufactured by Konica Minolta Sensing Inc.). However, amethod of measuring the irradiation peak intensity is not limitedthereto, and well-known measurement methods of the related art can beused.

In addition, it is preferable that the ultraviolet ray-curable ink becurable by irradiation of ultraviolet rays having an irradiation energyof 200 mJ/cm² or less. By using the ultraviolet ray-curable ink in therecording method according to the embodiment, even when an LED having arelatively low irradiation energy amount, the ink is curable; the heatradiation of the LED can be reduced; and high-speed printing can berealized at a low cost. The lower limit of an irradiation energy thatcan cure the ink is not particularly limited, but is preferably greaterthan or equal to 100 mJ/cm².

In addition, in order to suppress heat radiation caused by irradiation,the irradiation energy during recording is preferably less than or equalto 600 mJ/cm² and more preferably less than or equal to 500 mJ/cm². Thelower limit of the irradiation energy during recording is notparticularly limited, but is preferably greater than or equal to 200mJ/cm² in order to cure the ink sufficiently. When irradiation isperformed multiple times, the irradiation energy during recording is thetotal irradiation energy of all the irradiation energy amounts.

The irradiation energy described in this specification is calculated bymultiplying the irradiation peak intensity by the time from start tofinish of the irradiation. When irradiation is performed multiple times,the irradiation energy is the total irradiation energy of multiple timesof irradiation amounts. Single or plural emission peak wavelengths maybe present in the above-described preferable wavelength range. Whenplural emission peak wavelengths are present therein, the totalirradiation energy amount of ultraviolet rays having the emission peakwavelengths is set to the above-described irradiation energy.

Such an ink can be obtained by containing at least one of aphotopolymerization initiator which is decomposed by irradiation ofultraviolet rays having the above-described wavelength range; and apolymerizable compound which causes to start polymerization due toirradiation of ultraviolet rays having the above-described wavelengthrange.

In addition, the amount of the ink per unit surface area discharged ontothe recording medium (the amount of the ink attached) is preferably 5mg/inch² to 16 mg/inch².

In addition, the amount of the ink discharged per unit surface areavaries depending on the recording resolution and the amount of inkdischarged per recording unit region (pixel) which is defined by therecording resolution. However, when being expressed by “resolution insub-scanning direction×resolution in direction (main scanning direction)intersecting sub-scanning direction”, the recording resolution (printingresolution) is preferably 300 dpi×300 dpi to 1500 dpi×1500 dpi. It ispreferable that the nozzle density of the head and the amount of the inkdischarged be adjusted according to this recording resolution.

The amount of the ink discharged per pixel is preferably 2 ng/pixel to50 ng/pixel and more preferably 3 ng/pixel to 20 ng/pixel. In addition,the nozzle density (the distance between nozzles of a nozzle array) ispreferably 180 dpi to 720 dpi and more preferably 300 dpi to 720 dpi.

As described above, according to the embodiment, it is possible toprovide an ink jet recording method which has effects of obtainingsuperior curability and discharge stability; suppressing an increase inthe internal temperature of a recording apparatus after continuousprinting; and suppressing curing wrinkles. Furthermore, in the recordingmethod according to the embodiment, even when an ultraviolet ray-curableink having a low viscosity is used, an increase in the internaltemperature of a recording apparatus after continuous printing can besuppressed while securing superior curability and discharge stability.

Ink Jet Recording Apparatus

An embodiment of the invention relates to an ink jet recordingapparatus, that is, an ink jet printer. The recording apparatus is usedin the above-described ink jet recording method according to theembodiment. Hereinafter, the recording apparatus (printer) according tothe embodiment for realizing the recording method will be described indetail with reference to the drawings. However, the scope of theembodiments is not limited to the following drawings.

FIG. 1 is a block diagram illustrating a configuration example of theink jet recording apparatus according to the embodiment. A printerdriver is installed on a computer 130, and the computer 130 outputsprinting data corresponding to an image a printer 1 to form an image onthe printer 1. The printer 1 includes a transfer unit 20, a head unit30, an irradiation unit 40, a detection array 110, a memory 123, aninterface (I/F) 121, and a controller 120. The printer 1 that receivesprinting data from the computer 130, which is an external device,controls the respective units through the controller 120; and forms animage on a recording medium according to the printing data. The state inthe printer 1 is monitored by a detector array 110, and the detectorarray 110 outputs a detection result to the controller 120. Thecontroller 120 controls the respective units based on the detectionresult output from the detection array 110. The controller 120 storesprinting data, input through the interface 121, in the memory 123; andincludes a CPU 122 and a unit control circuit 124. Control informationfor controlling the respective units is also stored in the memory 123.

The printer according to the embodiment is a printer that forms variouscolor images on a recording medium, for example, a printer that formsimages using four color inks of CMYK (cyan, magenta, yellow, and black)or forms an underlying image, which gives superior opacifying propertiesto a recording medium, using white ink.

Examples of the kind of the printer according to the embodiment includea line printer and a serial printer. Both printers can be used as theprinter. The types of these printers are different.

The line printer which is a line type ink jet recording apparatusincludes a line head having a length longer than or equal to a width ofthe recording medium. While the line head and the recording medium moverelative to each other in a scanning direction intersecting the widthdirection, an ink is discharged from the line head onto the recordingmedium, that is, onto the recording medium which is scanned relativelyto the line head. In the line head, the head is fixed (substantially)without being moved and recording is performed in one pass (single pass)The line printer is superior to the serial printer from the viewpoint ofa higher recording speed.

“The line head having a length longer than or equal to a width of therecording medium” described above is not limited to a case in which thewidth of the recording medium and the length (width) of the line headare completely the same as each other. The width of the recording mediumand the length (width) of the line head may be different from eachother, for example, in a case in which the length (width) of the linehead corresponds to the width (recording width) of a recording mediumonto which the ink is discharged (on which an image is recorded).

On the other hand, in the serial printer which is a serial type ink jetrecording apparatus, while a head moves in a main scanning directionintersecting a sub scanning direction of a recording medium, mainscanning (pass) of discharging ink is performed. In this way, recordingis performed normally in two or more passes (multiple passes)

Ink Jet Head

The head unit 30 included in the ink jet recording apparatus (printer 1)includes a head (ink jet head) that discharges the ultravioletray-curable ink onto a recording medium to perform recording thereon.The head includes a cavity that accommodates the ink and causes the inkto be discharged from a nozzle; a discharge driving portion that isprovided for each cavity and gives a driving force to the ink to bedischarged; and the nozzle that is provided for each cavity. The cavityand the discharge driving portion and the nozzle that are provided foreach cavity may be provided in plural in a single head, independently ofeach other. The discharge driving portion is formed of anelectromechanical transducer element such as a piezoelectric elementthat changes the volume of the cavity with a mechanical deformation; oran electrothermal transducer element that heats the ink to generatebubbles in the ink and to discharge the ink. In the ink jet recordingapparatus, one or plural heads may be provided for each color ink. Whenplural heads are provided, the plural heads may be arranged in parallelin the width direction of a recording medium to constitute a line head.In this case, the above-described recording width can be increased. Whenrecording is performed using plural color inks, the ink jet recordingapparatus includes a head for each ink. The head can be constituted asillustrated in FIG. 3 and the like of JP-A-2009-279830.

Hereinafter, the line printer, which is an example of the printeraccording to the embodiment, will be described in detail referring toFIG. 2. In FIG. 2 used for the following description, the scale size ofeach member is appropriately changed so as to make the size of eachmember recognizable.

Line Printer

FIG. 2 is a cross-sectional view schematically illustrating an exampleof a configuration in the vicinity of the head unit, the transport unit,and the irradiation unit of the above-described line printer which is anexample of the printer according to the embodiment.

Transport rollers including an upper stream roller 25A and a lowerstream roller 25B are rotated by a transport roller (not illustrated),and thus a transport drum 26 is driven. Along with the rotation of thetransport rollers, a recording medium S is transported alongcircumferential surfaces of the transport rollers 25A and 25B and thetransport drum 26 which is a support. In the vicinity of the transportdrum 26, line heads including a head K, a head C, a head M, and a head Yare arranged opposite the transport drum 26.

The support has a surface on which the recording medium S istransported, supports the recording medium S, and moves relative to theheads. In FIG. 2, the transport drum 26 that corresponds to the supporthas a surface on which the recording medium S is transported, supportsthe recording medium S, moves relative to the heads, and passes througha position opposite each line head. When the support moves relatively tothe heads while supporting the recording medium S, a time (period) inwhich the supports moves from an arbitrary position and returns to theposition is preferably longer than or equal to 5 seconds and morepreferably 6 seconds. When the time is in the above-described range, anincrease in temperature caused by the heat radiation of the support canbe suppressed. In addition, the upper limit of the period is notparticularly limited, but is preferably, for example, shorter than 15seconds in order to realize high-speed printing.

The movement of the support within a predetermined period only has to beperformed at least while ink jet recording is performed; and only has tobe continuously or intermittently performed while ink jet recording isperformed.

The shape of the support is not limited to a drum shape illustrated inFIG. 2. Preferable examples thereof include a drum shape, a rollershape, and a plate shape (for example, a platen) that supports therecording medium S. However, the shape of the support is not limitedthereto. The relative movement of the support to the heads may beperformed in a method in which the support moves (rotates) from aposition in a single direction and returns to the same position; or in amethod in which the support moves from a position in a first directionand returns to the same position in a second direction different fromthe first direction. In the latter method, for example, a configurationmay be adopted in which the movement in the first direction is performedwhile recording is performed on a sheet of recording medium; and themovement in the second direction is performed in order to performrecording on the next sheet of recording medium after finishingrecording on the previous sheet of recording medium.

In the case of a serial printer, the movement in the first directioncorresponds to sub-scanning. In addition, the relative movement of thesupport to the heads includes a case in which the heads move relativelyto the support.

Examples of a material of the support include metals, resins, andrubbers. Among these, metals are preferable. However, the material ofthe support is not limited thereto. When the material is a metal, unlikepolymer materials such as rubber, cracks which are considered to be adeterioration caused by heat are not generated even after the support isused for a long period of time. Therefore, the support can be used for along period of time. Examples of the metal include aluminum, stainlesssteel, copper, and iron; and alloys thereof. However, the metal is notlimited thereto. Furthermore, a metal surface of the support, that is, atransport surface of the recording medium may be coated with a coatingagent or the like. As a result, the hardness of the surface of thesupport can be improved compared to a non-coated support; and arecording medium is not easily slipped on the support. Examples of thecoating agent include organic coating agents such as resins, inorganiccoating agents such as inorganic compounds, and complex coating agentsthereof. However, the coating agent is not limited thereto. Theabove-described features relating to the support is also applicable tothe serial printer as well as the line printer.

In this way, recording is performed through the discharge operation ofdischarging the ink to the recording medium S opposite each line headand attaching the ink thereon. Preliminary curing irradiation portions42 a, 42 b, 42 c, and 42 d are arranged downstream of each line head inthe transport direction to irradiate the recording medium S withultraviolet rays. A main curing irradiation portion 44 is arrangedfurther downstream thereof in the transport direction. Such a recordingapparatus can be constituted as illustrated in FIG. 11 ofJP-A-2010-269471.

In this specification, “preliminary curing” refers to the ink beingpreliminarily cured (pinned), more specifically, to the ink beingpreliminarily cured before main curing in order to prevent bleedingbetween dots and to control the dot diameter. In general, thepolymerization degree of polymerizable compounds in the preliminarycuring is lower than that of the polymerizable compounds in the maincuring which is performed after the preliminary curing. In addition,“main curing” refers to dots, which are formed on a recording medium,being cured to a degree that is required for using a recorded material.In this specification, “curing” refers to the main curing unlessspecified otherwise.

The ink only has to be cured by irradiation of ultraviolet rays emittedfrom the main curing irradiation portion 44. Therefore, the curingoperation may be finished by irradiation of only the main curingirradiation portion 44 without irradiating ultraviolet rays from a partor all of the preliminary curing irradiation portions 42 a, 42 b, 42 c,42 d. In this way, the curing operation may be performed by only themain curing without the preliminary curing.

As described above, according to the embodiment, it is possible toprovide an ink jet recording apparatus which has effects of obtainingsuperior curability and discharge stability; suppressing an increase inthe internal temperature of a recording apparatus after continuousprinting; and suppressing curing wrinkles. Furthermore, in the recordingapparatus according to the embodiment, even when an ultravioletray-curable ink having a low viscosity is used, an increase in theinternal temperature of a recording apparatus after continuous printingcan be suppressed while securing superior curability and dischargestability.

EXAMPLES

Hereinafter, the embodiments will be described in detail using Examplesand Comparative Examples. However, the embodiments are not limited toExamples and Comparative Examples.

Used Materials

Materials used in Examples and Comparative Examples are as follows.

Polymerizable Compounds

-   -   VEEA (trade name, 2-(2-vinyloxyethoxy)ethyl acrylate,        manufactured by Nippon Shokubai Co., Ltd., monofunctional        (meth)acrylate    -   NEW FRONTIER PHE (trade name, phenoxyethyl acrylate,        manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd, monofunctional        (meth)acrylate; hereinafter, referred to as “PEA”)    -   APG-100 (trade name, dipropylene glycol diacrylate, manufactured        by Shin-Nakamura Chemical Co., Ltd., bifunctional        (meth)acrylate; hereinafter, referred to as “DPGDA”)    -   A-DPH (trade name, tripropylene glycol diacrylate, manufactured        by Shin-Nakamura Chemical Co., Ltd., bifunctional (meth)        acrylate)

Photopolymerization Initiator

-   -   DAROCUR TOP (trade name,        2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, manufactured by        BASF Japan Ltd.; hereinafter, referred to as “TPO”)    -   IRGACURE 819 (trade name,        bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, manufactured        by BASF Japan Ltd.; hereinafter, referred to as “819”)

Colorant

-   -   CYANINE BLUE KRO (trade name, C.I. Pigment blue 15:3        (phthalocyanine pigment), manufactured by Sanyo Color Works        Ltd., pigment average particle size: 80 nm; hereinafter,        referred to as “PB 15:3”)

Dispersant

-   -   SOLSPERSE 32000 (trade name, manufactured by Avecia Co.;        hereinafter, referred to as “32000”) Preparation of Ultraviolet        Ray-Curable Inks 1 to 11

Materials shown in Table 1 below were added according to contentsthereof (unit: % by mass) shown in Table 1, followed by stirring with ahigh-speed water-cooling type stirrer. As a result, ultravioletray-curable inks 1 to 11 were obtained. The viscosity of each ink wasset to a desired value according to the above-described viscositysetting method.

Measurement and Evaluation Items 1. Average Equivalent of PolymerizableUnsaturated Double Bond of Ink

The average equivalent of polymerizable unsaturated double bond of theink was obtained according to the above-described expressions (2) and(3).

The evaluation criteria are as follows. The evaluation results includingmeasured values and the following ranks (criteria) are shown in Table 1.

1: From 100 to 150

2: Less than 1003: Greater than 150

2. Measurement (Rank) of Viscosity at 20° C. of Ink

Using a DVM-E type rotary viscometer (manufactured by Tokyo Keiki Inc.)the viscosity of each ink prepared above was measured under conditionsof 20° C. and a rotating speed of 10 rpm.

As a rotor, a DVM-E type cone rotor having a cone angle of 1° 34′ and acone radius of 2.4 cm was used.

The evaluation criteria are as follows. The evaluation results are shownin Table 1 below.

1: Less than 15 mPa·s2: From 15 mPa·s to 25 mPa·s3: Greater than 25 mPa·s

3. Evaluation for Curability of Ink

Each ink prepared above was coated on a PET film (PET 50 (K2411) PA-Tl8LK (trade name), manufactured by Lintec Corporation) using a barcoater. As a result, a coating film having a thickness of 10 μm wasobtained. The obtained coating film was irradiated with ultraviolet rayshaving an irradiation intensity of 1,000 mW/cm² and a peak wavelength of395 nm for a predetermined time using an ultraviolet ray irradiationdevice (UV-LED). As a result, the coating film was cured. In a fingertack test, it was determined that an image (surface of the coating film)was cured when tackiness was disappeared.

The evaluation was performed by calculating an irradiation energy ofultraviolet rays required for curing. The irradiation energy (mJ/cm²) isobtained from a product obtained by measuring an irradiation intensity(mW/cm²) of ultraviolet rays, emitted from a light source to anirradiation surface; and multiplying the irradiation intensity (mW/cm²)by an irradiation duration time (s). The irradiation intensity wasmeasured using an ultraviolet ray intensity meter UM-10 and a lightreceiving unit UM-400 (all of which are manufactured by Konica MinoltaSensing Inc.).

The evaluation criteria are as follows. The evaluation results thereofare shown in Table 1 below.

A: The image was cured at an integrated light intensity of 200 mJ/cm² orlessB: The image was cured at an integrated light intensity exceeding 200mJ/cm²

4. Evaluation for Storage Stability of Ink

Each ink, of which the viscosity was measured above in the item “2.”,was poured into a 50 mL glass bottle. The glass bottle was hermeticallysealed, was put into a constant-temperature bath at 60° C., and was leftto stand for 1 week. Then, the temperature was lowered to roomtemperature, and the viscosity of the ink was measured in the samemethod as in the item “2.”. The storage stability was evaluated based onthe viscosity ratio before and after storage (ratio of the viscosity ofthe ink before storage to the viscosity of the ink after storage).

The evaluation criteria are as follows. The evaluation results thereofare shown in Table 1.

A: The viscosity ratio was less than 5%B: The viscosity ratio was greater than or equal to 5%

TABLE 1 Ultraviolet Ray-Curable Ink Ink No. of Ultraviolet Ray-CurableInk Material Abbreviation 1 2 3 4 5 6 7 8 9 10 11 Polymerizable VEEA 2020 30 40 70 75 7 28 38 5 60 Compounds PEA 40 20 20 10 — 5 55 60 10 9 —DPGDA 20 36 30 30 10 — 26 — 40 70 — A-DPH 8 12 8 8 8 8 — — — 4 28Photopolymerization 819 3 3 3 3 3 3 3 3 3 3 3 Initiator TPO 5 5 5 5 5 55 5 5 5 5 Dispersant 32000 1 1 1 1 1 1 1 1 1 1 1 Colorant PB15:3 3 3 3 33 3 3 3 3 3 3 Total 100 100 100 100 100 100 100 100 100 100 100 AverageEquivalent of Polymerizable 1 1 1 1 2 2 3 3 1 1 2 Unsaturated DoubleBond (Rank) Viscosity at 20° C. (Rank) 2 2 2 2 1 1 3 2 1 3 2 CurabilityA A A A A A B B A B A Storage Stability A A A B B B A A A A B

The ultraviolet ray-curable inks 1 to 4 and 9 correspond to the inksused in Examples; and the ultraviolet ray-curable inks 5 to 8, 10, and11 correspond to the inks used in Comparative Examples.

Hereinafter, recording methods according to the respective Examples andComparative Examples will be described.

Example 1

The line printer illustrated in FIG. 2 was used in which four line headshaving a length substantially equal to a width (recording width) of arecording surface, on which an image is to be recorded, were arranged inparallel in the width direction. Among the heads and light sources inFIG. 2, the head C and the light sources 42 b and 44 were used; and theother heads and light sources were not used.

The transport drum 26 was made of aluminum; the diameter of thetransport 26 was 500 mm; the printing speed was 285 mm/s; and the drumrotation period was 5.5 s.

The ink 1 shown in Table 2 was continuously discharged for 10 minutes(continuous printing was performed for 10 minutes) from the head Chaving a nozzle density of 600 dpi onto the PET film (PET 50 (K2411)PA-Tl 8LK) under conditions of a recording resolution of 600 dpi×600 dpiand one pass (single pass). The amount of ink droplets per pixel wasadjusted such that the thickness of the cured film was 11 μm. In Example1, the amount of ink droplets was 7 ng. In this way, a solid patternimage was formed. The solid pattern image refers to an image in whichdots are formed on all the pixels of an image (one pixel is the minimumrecording unit region defined by the recording resolution).

The temperature during discharge was 35° C. as shown in the item“Discharge Temperature” of Table 2. The discharge temperature describedherein was measured as follows. An ink heating device was arranged on apath for supplying the ink to the head C, and the ink heated to adesired temperature was supplied to the head C. The temperature of athermocouple provided on a nozzle surface of the head C, was measuredbefore starting printing, and a measured temperature is set to thedischarge temperature. The nozzle surface (nozzle plate) was made ofstainless steel.

Next, the ink attached onto the PET film was irradiated with ultravioletrays to cure the ink. Specifically, first, an LED having a peakwavelength of 395 nm and an irradiation peak intensity of 500 mW/cm² wasused as the light source 42 b. Ultraviolet rays having an irradiationenergy of 20 mJ/cm2 was emitted from the LED to preliminarily cure theink. An LED having a peak wavelength of 395 nm and an irradiation peakintensity of 1,500 mW/cm2 was used as the light source 44. Ultravioletrays having an irradiation energy of 400 mJ/cm2 was emitted from the LEDfor a predetermined time to cure the solid pattern image. In this way,the solid pattern image was cured and thus a recorded material isobtained. In a finger tack test, it was confirmed that the tackiness ofthe image (surface of the coating film) was disappeared.

Examples 2 to 7 and Comparative Examples 1 to 12

A recorded material was obtained in the same method as that of Example1, except that the used ink and the discharge temperature wererespectively changed according to Tables 2 and 3 below.

Example 8

A recorded material was obtained in the same method as that of Example1, except that the diameter of the transport drum 26 was 318 mm and thedrum rotation period was 3.5 s (printing speed was the same as that ofExample 1).

Example 9

A recorded material was obtained in the same method as that of Example1, except that a metal halide lamp (in Table 2, abbreviated as “MHL”)was used as the light source 42 b instead of a LED.

Example 10

A recorded material was obtained in the same method as that of Example1, except that a serial printer in which an LED having a peak intensityof 500 mW/cm² was mounted adjacent to a carriage as a light source wasused instead of the line printer. The used serial printer was an ink jetprinter illustrated in FIG. 2 of JP-A-2010-167677. A nozzle array of ahead (serial head) was filled with the ink 1. Dots were formed on thesame recording region of the PET film in 4 passes (2 passes in the mainscanning direction×2 passes in the sub-scanning direction) underconditions of a nozzle density of the head of 300 dpi; an amount of inkdroplets of 7 ng; a recording resolution of 600 dpi×600 dpi (a recordingresolution per 1 pass of 300 dpi×300 dpi); and a discharge temperatureof 35° C. As a result, a solid pattern image having a thickness of 11 μmwas formed.

The same light source 42 b and LED mounted on the above-described lineprinter were provided on the carriage of the printer so as to have thesame length as that in the sub-scanning direction of the head. Theirradiation energy per pass was 40 mJ/cm², and preliminary curing wasperformed for each pass. The same LED as the light source 44 mounted onthe above-described line printer were provided in the width directionand downstream of the carriage of the printer in the transport directionof the recording medium so as to have the same length as that of therecording medium. The solid image of the recording medium, which wastransported to the downstream side of the carriage in the transportdirection of the recording medium, was irradiated with ultraviolet rayshaving an irradiation energy of 400 mJ/cm2 to cure the solid image. As aresult, the solid image was cured and thus a recorded material wasobtained. Similarly to the case of Example 1, in a finger tack test, itwas confirmed that the tackiness of the image (surface of the coatingfilm) was disappeared.

Measurement and Evaluation Items 5. Evaluation for Discharge Stability

Using each ink having the number show in Tables 2 and 3, printing wascontinuous performed for 10 minutes (the ink was continuously dischargedfrom 300 nozzles for 10 minutes). Before starting printing, 500 dropletsof the ink were further discharged from the nozzles to measure theamount (mass) of one droplet discharged. Immediately after finishingprinting, the amount (mass) of one droplet discharged was measured inthe same method as that before printing.

The evaluation criteria are as follows. The evaluation results thereofare shown in Tables 2 and 3.

A: During continuous printing, there were no nozzles which did notdischarge droplets. The change of the amount of one droplet afterfinishing printing to the amount of one droplet before starting printingwas less than or equal to 5%B: During continuous printing, there were no nozzles which did notdischarge droplets. The change of the amount of one droplet afterfinishing printing to the amount of one droplet before starting printingwas greater than 5%C: During continuous printing, there were nozzles which did notdischarge droplets

The following items “6.” and “7.” relate to the evaluation for anincrease of the internal temperature of a recording apparatus aftercontinuous printing. In order to improve the reliability of theevaluation results, the evaluation is performed at plural points in therecording apparatus. As the plural points of the recording apparatus,the nozzle surface of the nozzle plate on the head side and the drum onthe recording medium were selected.

6. Evaluation for Increase on Nozzle Surface of Head after ContinuousPrinting

Before starting printing, the temperature of the thermocouple, providedin the nozzle surface of the nozzle plate, was measured. Aftercontinuously performing printing for 10 minutes, the temperature of thethermocouple was measured. The difference between the temperature beforestarting printing and the temperature after continuous printing wasevaluated as an increase in the temperature of the nozzle surface of thehead after continuous printing.

The evaluation criteria are as follows. The evaluation results thereofare shown in the abbreviation “Temperature Increase of Nozzle SurfaceAfter Continuous Printing” of Tables 2 and 3.

A: Lower than 10° C.B: 10° C. or higher and lower than 15° C.C: 15° C. or higher7. Evaluation for Increase of Drum after Continuous Printing

Examples and Comparative Examples other than Example 10 were evaluatedas follows. The surface temperature of a position of the surface of thetransport drum opposite the head was measured before starting printing.After continuously performing printing for 10 minutes, the surfacetemperature of the transport drum opposite the head was measured in thesame method as that before starting printing. The difference between thesurface temperature before staring printing and the surface temperatureafter continuous printing was evaluated as an increase in thetemperature of the transport drum after continuous printing.

In Example 10, the evaluation was performed in the same method as thatof the other Example and Comparative Examples, except that the surfacetemperatures of a platen were measured before starting printing andafter continuous printing instead of that of the transport drum oppositethe head.

The evaluation criteria are as follows. The evaluation results thereofare shown in the abbreviation “Temperature Increase of Drum AfterContinuous Printing” of Tables 2 and 3. In Example 10, to be exact,“Temperature Increase of Drum After Continuous Printing” refers to anincrease in the temperature of the platen after continuous printing.

A: Lower than 10° C.B: 10° C. or higher and lower than 15° C.C: 15° C. or higher

8. Evaluation for Curing Wrinkles

Regarding each of the recorded materials obtained in Examples andComparative Examples, the surface of the cured film (cured coating film)was visually inspected. The evaluation criteria are as follows. Theevaluation results thereof are shown in Tables 2 and 3.

A: No wrinkles were observedB: Wrinkles are observed on a partial region of the cured filmC: Wrinkles are observed on the entire surface of the cured film

TABLE 2 Examples Item/Example No. 1 2 3 4 5 6 7 8 9 10 Ink No. 1 1 1 2 34 9 1 1 1 Average Equivalent of Polymerizable 1 1 1 1 1 1 1 1 1 1Unsaturated Double bond Viscosity at 20° C. (Rank) 2 2 2 2 2 2 1 2 2 2Discharge Temperature (° C.) 35 30 40 35 35 35 35 35 35 35 Curing LightSource LED LED LED LED LED LED LED LED MHL LED Drum Rotation Period (s)5.5 5.5 5.5 5.5 5.5 5.5 5.5 3.5 5.5 Serial Discharge Stability A A A A AA A A A A Temperature Increase of Nozzle Surface 4° C. 2° C. 7° C. 6° C.6° C. 7° C. 4° C.  7° C.  6° C. 2° C. After Continuous PrintingTemperature Increase of Drum After 6° C. 3° C. 8° C. 7° C. 7° C. 9° C.5° C. 13° C. 11° C. 4° C. Continuous Printing Curing Wrinkles A A A A AA C A A A

TABLE 3 Comparative Examples Item/Comparative Example No. 1 2 3 4 5 6 78 9 10 11 12 Ink No. 1 1 5 5 5 7 7 7 10 6 11 8 Average Equivalent ofPolymerizable 1 1 2 2 2 3 3 3 1 2 2 3 Unsaturated Double bond Viscosityat 20° C. (Rank) 2 2 1 1 1 3 3 3 3 1 2 2 Discharge Temperature (° C.) 4328 40 43 28 40 43 28 40 40 40 40 Curing Light Source LED LED LED LED LEDLED LED LED LED LED LED LED Drum Rotation Period (s) 5.5 5.5 5.5 5.5 5.55.5 5.5 5.5 5.5 5.5 5.5 5.5 Discharge Stability B C B B B C B C C B B ATemperature Increase of Nozzle Surface 13° C. 3° C. 17° C. 20° C. 11° C.4° C. 12° C. 2° C. 4° C. 17° C. 17° C. 4° C. After Continuous PrintingTemperature Increase of Drum After 12° C. 4° C. 16° C. 16° C. 13° C. 6°C. 11° C. 3° C. 6° C. 16° C. 16° C. 6° C. Continuous Printing CuringWrinkles A A C C C A A A A C B B

It was found from the above results that superior curability anddischarge stability were obtained; the temperature increase aftercontinuous printing was suppressed; and curing wrinkles are suppressedwhen a specific ink jet recording method (Examples) is compared to theother recording methods (Comparative Examples), the specific ink jetrecording method including: the discharging step of discharging anultraviolet ray-curable ink, which has a viscosity at 20° C. of 25 mPa·sor less and an average equivalent of polymerizable unsaturated doublebond of 100 to 150, from a head onto a recording medium at a dischargetemperature of 30° C. to 40° C.; and the curing step of irradiating theultraviolet ray-curable ink, attached onto the recording medium, withultraviolet rays emitted from a light source to cure the ultravioletray-curable ink. Hereinafter, each of Examples and Comparative Exampleswill be discussed. However, the scope of the embodiments is not limitedto the following discussion.

First, when the viscosity at 20° C. of the ultraviolet ray-curable inkwas less than or equal to 25 mPa·s, curability was superior. Inaddition, when the average equivalent of polymerizable unsaturateddouble bond of the ultraviolet ray-curable ink was greater than or equalto 100, the storage stability of the ink was superior. Furthermore, whenthe average equivalent was less than or equal to 150, the curability ofthe ink was superior. Therefore, in the case of actual recording inwhich plural color inks are used, the irradiation energy of the lightsource 42 b, which is required to perform the minimum curing forpreventing the color inks from being mixed, can be reduced. As a result,an increase in the temperature of the transport drum 26, caused by theirradiation and heat radiation of the light source 42 b, can be reduced.Therefore, the above-described configuration is assumed to be preferablein the embodiments.

In addition, when the average equivalent of polymerizable unsaturateddouble bond of the ultraviolet ray-curable ink was less than 100; orwhen the discharge temperature was higher than 40° C., the increase inthe temperature of the nozzle surface of the head after continuousprinting and the increase in the temperature of the transport drum aftercontinuous printing were significant. More specifically, it is assumedthat, when the average equivalent of polymerizable unsaturated doublebond of the ultraviolet ray-curable ink was less than 100, the amount ofreaction heat generated during curing was large; and as a result, theinternal temperature of the recording apparatus after continuousprinting was significantly increased.

When the temperature of the transport drum after continuous printing issignificantly increased, the recording medium may be thermally deformed,which may cause a problem in that the quality of an obtained recordedmaterial deteriorates. Meanwhile, when the temperature of the nozzlesurface of the head after continuous printing is significantlyincreased, the amount of ink droplets discharged is largely changed,which may cause a problem in that image stability deteriorates. On theother hand, according to Examples, an increase in both temperatures ofthe transport drum and the nozzle surface can be effectively suppressed.Therefore, the above-described problems do not occur.

In addition, in Example 8 in which the drum rotation period (s) wasshorter than that of the other examples, particularly, the temperatureof the transport drum after continuous printing was further increased.

In addition, in Example 9 in which the metal halide lamp was used as thecuring light source instead of LED, the temperature of the transportdrum after continuous printing was further increased. The reason isconsidered to be that the temperature of the transport drum wassignificantly increased due to heat radiation of the metal halide lamp.When a metal halide lamp is used as a light source, there are cases inwhich the temperature of the recording medium may be thermally deformeddue to an increase in the temperature of the transport drum caused bythe heat radiation; or an installment size may be necessary because themetal halide lamp has a larger size than that of an LED. That is, it ispreferable that an LED be used from the viewpoints of effectivelysuppressing an increase in the temperature of the transport drum aftercontinuous printing; and of obtaining a low-heat-generating andspace-saving recording apparatus.

In addition, in Example 10 in which the serial printer was used insteadof the line printer, the temperature increase is less than that of theline printer; whereas the recording speed is lower than that of the lineprinter. That is, in the recording method according to the embodiment,it is assumed that recording capable of effectively suppressing curingwrinkles can be performed using a line printer even at a high speed.

Although not shown in the examples, as the irradiation peak intensity ofthe light source (LED) 42 b was higher, curing wrinkles were suppressedmore effectively. In particular, when the ink discharged onto therecording medium was irradiated with ultraviolet rays, the initialirradiation peak intensity of the ultraviolet rays was set to bepreferably greater than or equal to 500 mW/cm² and more preferablygreater than or equal to 800 mW/cm². As a result, curing wrinkles weresuppressed more effectively.

Although not shown in the examples, a recorded material was obtained inthe same method as that of Example 1, except that a material of halfportions of the transport drum on the surface side of the drum otherthan portions on a shaft side of the transport drum were made of rubbernot aluminum. After a long period of use (6 months), cracks weregenerated. The reason is considered to be that the rubber surface of thetransport drum deteriorated due to heat. Therefore, it is consideredthat a metal is preferable as the material of the transport drum.

The entire disclosure of Japanese Patent Application No. 2012-102535,filed Apr. 27, 2012 and 2012-250029, filed Nov. 14, 2012 are expresslyincorporated by reference herein.

1. An ink jet recording method comprising: discharging an ultravioletray-curable ink, which has a viscosity at 20° C. of 25 mPa·s or less andan average equivalent of polymerizable unsaturated double bond of 100 to150, from a head onto a recording medium at a discharge temperature of30° C. to 40° C.; and irradiating the ultraviolet ray-curable ink,attached onto the recording medium, with ultraviolet rays emitted from alight source to cure the ultraviolet ray-curable ink, wherein theultraviolet ray-curable ink comprises a monofunctional (meth) acrylateselected from the group consisting of phenoxyethyl (meth) acrylate,benzyl (meth) acrylate, 2-hydroxyphenoxypropyl (meth) acrylate, andphenoxy diethylene glycol (meth) acrylate.
 2. The ink jet recordingmethod according to claim 1, wherein the head is a line head having alength longer than or equal to a width of the recording medium, andrecording is performed using a line type ink jet recording apparatusthat discharges the ultraviolet ray-curable ink from the line head ontothe recording medium which is scanned relative to the line head.
 3. Theink jet recording method according to claim 1, wherein a support has asurface on which the recording medium is transported, supports therecording medium, and moves relative to the head, and a time period inwhich the support moves relative to the head from a position and returnsto the position is longer than or equal to 5 seconds.
 4. The ink jetrecording method according to claim 3, wherein a material of the supportfor the recording medium is a metal.
 5. The ink jet recording methodaccording to claim 1, wherein a viscosity at 20° C. of the ultravioletray-curable ink is 15 mPa·s to 25 mPa·s.
 6. The ink jet recording methodaccording to claim 1, wherein the light source is a light emittingdiode.
 7. The ink jet recording method according to claim 1, wherein theultraviolet ray-curable ink contains 30% by mass to 70% by mass ofmonofunctional (meth)acrylate and 20% by mass to 60% by mass ofbifunctional or higher polyfunctional (meth)acrylate.
 8. The ink jetrecording method according to claim 1, wherein the ultravioletray-curable ink is curable by irradiation of ultraviolet rays having anirradiation energy of 200 mJ/cm2 or lower.
 9. An ink jet recordingapparatus which performs recording using the ink jet recording methodaccording to claim
 1. 10. An ink jet recording apparatus which performsrecording using the ink jet recording method according to claim
 2. 11.An ink jet recording apparatus which performs recording using the inkjet recording method according to claim
 3. 12. An ink jet recordingapparatus which performs recording using the ink jet recording methodaccording to claim
 4. 13. An ink jet recording apparatus which performsrecording using the ink jet recording method according to claim
 5. 14.An ink jet recording apparatus which performs recording using the inkjet recording method according to claim
 6. 15. An ink jet recordingapparatus which performs recording using the ink jet recording methodaccording to claim
 7. 16. An ink jet recording apparatus which performsrecording using the ink jet recording method according to claim
 8. 17.The ink jet recording method according to claim 1, wherein a content ofthe monofunctional (meth) acrylate is 30% by mass to 70% by mass withrespect to the total mass of the ink composition.
 18. The ink jetrecording method according to claim 1, wherein the ultravioletray-curable ink further comprises a vinyl ether group-containing(meth)acrylic acid ester represented by formula (I):—CH₂═CR¹—COOR²—O—CH═CH—R³  (I) (in the formula, R¹ represents a hydrogenatom or a methyl group, R² represents a divalent organic residue having2 to 20 carbon atoms, and R³ represents a hydrogen atom or a monovalentorganic residue having 1 to 11 carbon atoms).